In addition to being a leading cause of food poisoning in the developed world, Salmonella typhimurium has been extensively studied as a model organism both for studies of intracellular pathogenesis and for basic biological studies. One group of Salmonella enzymes which has been less well studied than most is the esterases. Biochemical and genetic data indicate S. typhimurium produces at least 15 esterases. One of these, the product of the apeE gene, is located in the outer membrane and is not present in the closely related species E. coli. Its surface location and absence from E. coli suggest that it may play a role in Salmonella pathogenesis. Furthermore, apeE is repressed by the gene apeR. Although it is not known what signals apeR responds to , it has been shown that it effects the expression of other genes besides apeE. beta-galactosidase fusion has been isolated to one gene, ard-1, which is activated by apeR. This project will characterize the apeR regulatory system. l. The apeR gene will be cloned and sequenced and its sequence compared to other known regulatory genes. 2. Transcription of the apeE gene will be analyzed to identify the promoter. Possible promoter sequences will be cloned into an expression vector to determine the effect of apeR mutations on their transcription. 3. The environmental signals which affect apeE expression will be identified using an apeE::lacZ fusion. The level of beta-galactosidase expression will be assayed under various conditions. 4. Other genes which are regulated by apeR will be identified and characterized. First, ard-1 will be cloned and sequenced. Then beta- galactosidase fusions will be isolated to other genes whose expression is regulated by apeR. Preliminary sequence will be obtained to identify these genes and possibly to identify their apeR dependent promoters.